1. Field of the Invention
The present invention relates to a method of selectively hydrogenating and removing acetylenes in a C.sub.4 -hydrocarbon mixture containing a substantial amount of butadiene with the minimal loss of butadiene.
2. Prior Art
A large amount of butadiene has been produced by the dehydrogenation of butane and/or butene, or may be derived by cracking various gaseous materials such as waste gases recovered from petroleum refining processes or a variety of hydrocarbons. Irrespective of the recovered butadiene sources, the purity of the thus recovered butadienes does not reach the level as required for using as starting materials for other organic compounds. They contain vinylacetylene, ethylacetylene and methylacetylene (Hereinafter referred to as acetylenes.) which can vary hardly be separated or removed by the ordinary fractional distillation. For this reason, it has been conventionally practised to hydrogenate a C.sub.4 -stream containing butadiene and acetylenes prior to or after the removal of butane and butene which may be easily separated. Such conventional catalytic hydrogenation has been carried out on a variety of hydrogenation catalysts in a vapor or liquid phase. However, in the conventional vapor phase or liquid phase processes, the aimed hydrogenation is often accompanied with undesirable hydrogenation of butadiene and/or polymerization thereof to result in considerable loss of the butadiene product. Moreover, in the vapor phase process which are carried out at a relatively high temperature of higher than 150.degree. C., the activity of the catalyst is adversely lowered due to the side reaction, i.e. polymerization of butadiene. While if by the conventional liquid phase process acetylenes are hydrogenated to a very small amount level, a large quantity of butadiene will be lost due to the hydrogenation into monoolefins. On the other hand, if it is intended to suppress the loss of butadiene, the acetylene would be left unhydrogenated.
In fact, the amounts of acetylenes contained in the hydrocarbon mixture after hydrogenation may be controlled below the low level if the content of the acetylenes in the original starting hydrocarbon mixture is low, for example less than about 1,600 ppm. However, in such cases, the loss of butadiene would be increased inevitably if it is intended to improve the convertion ratio of acetylenes.
In the method disclosed in Japanese Patent Publication No. 2257/'63, a hydrocarbon mixture having a small acetylene contnet is used as starting material but the hydrogenation ratio of acetylenes is low as about 97% at best. Further, this prior art method is disadvantageous in that the hydrogenation reaction is carried out under the condition at which the partial pressure of hydrogen in the reaction system is high. If a hydrocarbon mixture of high acetylene content is used as starting material under such condition, the contained acetylenes can not be sufficiently removed as will be specifically shown hereinafter in the comparative examples.
Likewise in the method disclosed in Unexamined Japanese Patent Publication No. 95903/'74, the acetylene content after the hydrogenation reaction is higher than the order of hundreds ppm. We have examined and found that the acetylenes will not be removed sufficiently since the liquid reaction mixture is fed from the bottom of the reaction vessel upwardly and thus the catalytic reaction is carried out under the condition at which the reaction vessel is substantially full with liquid hydrocarbons although a gas mixture containing an inert gas and hydrogen is used in this prior art method.
Also in the method described in Japanese Patent Publication No. 19522/'75, the acetylene content after the hydrogenation reaction is higher than the order of hundreds ppm. In this prior art method, hydrogenation is effected while the liquid phase hydrocarbon being avoided to be saturated with hydrogen. However, this publication specifically describes that the liquid hydrocarbon is introduced from the lower portion of the reactor, and thus it is considered that the removal of the acetylenes would be insufficient likewise in the prior art methods described above. Although the Publication refers to the downward introduction of the liquified hydrocarbons briefly, but it fails to specifically describe in what manners of contact such downward introduction is used.